500 CHEMICAL ENGINEERING resulting in greater resistance to compression at furnace temperature but lessened resistance to sudden temperature changes and to slag action. Mixtures composed of a smaller proportion of flint clay and more of the plastic can be made molded into bricks by means of auger or piston presses. Shapes are almost invariably hand molded. The firing of the refractories is usually conducted in round, down draft or beehive kilns and the temperature reached may vary from that of cone 5 to cone 10 or 11. There has been a tendency in the past to carry the burning to a point not sufficiently high, with the result that the bricks shrink in use more than they should. However, this practice is being eliminated more and more and it is being realized that for general purposes the product should be fired as high as possible. The more economical continuous regenerating kilns are being used only in a few plants. Properties of Flint-clay Refractories.—These may be briefly summarized as follows: The refractoriness of No. 1 materials as indicated by the softening point is usually not lower than that of cone 31 (about 1,685°C. or 3,065°F.). Kanolt has found the mean melting point of 41 samples of firebrick to be 1,649°C., determined in the Arsem furnace. The permanent contraction or expansion of such products when heated uniformly in a suitable furnace to a temperature of 1,400°C., maintained for 5 hr., is as a rule not more than 1 per cent in terms of the original length. For high-grade materials the allowable contraction should not be more than 1 and the expansion not more than 0.5 per cent. For materials used for less severe heat duty softening temperature may be dropped to cone 28 and the reheating temperature to 1,350°C. The porosity of these products varies as a rule from 20 to 30 per cent and the specific gravity from 2.5 to 2.7. Products of first quality should be able to resist a pressure of 40 Ib. per square inch at a maximum temperature of 1,350°C., with a compression of not more than 5.5 per cent in terms of the original length. The specific heat possesses a value close to 0.2. According to Mellor the specific heat of firebrick varies with temperature according to the relation: 8 = 0.193 +• 0.00006Z, where S = mean specific heat and t = temperature in degrees Centigrade.1 The thermal conductivity K* according to Heyn increases with temperature (K gives when divided by 100,000, cal. per second through 1 cm.3 per 1°C., difference in temperature between the faces). Thus for 200°C., K = 145; for 400°, K = 180; for 600°, K = 220; for 880°, K = 240; for 1000°, -K « 260; and for 1100°, K = 270. In general the conductivity varies inversely with porosity though the size of the pore spaces is manifestly of considerable importance. It would be expected also that this property is altered by reheating. From data published by the Celite Products Co. it appears that the thermal conductivity of average flint clay fire brick is about 0.002064 cal. per cubic centimeter per second, per degree Centigrade, at 149°C.; 0.00275 at 538°C. and 0.003784 at 1,037°C. Expressed in B.t.u., per hour, per square foot, through 1 cu. in. and per degree Fahrenheit temperature difference the heat conductivity would be approximately 5.8, at 200°F., 7 at 600°, 9 at 1,200° and 12 at 2,200°F. Horning' reports the B.t.u. transmitted per 24 hr., through 1 sq. ft. and 1 in. thickness per degree Fahrenheit tenperature difference, to be 92 at 300° F. for dense and 62 for porous firebrick. At 1,000°F. the same value is 116 for the dense and 82 for the porous brick. The data available relating to thermal expansion are meagre and where given no statement is made as to the character of the firebrick in question, whether porous or 1 Trans. English Ceram. Soc., Vol. 12, p. 279. 2 Stahl u. Eisen, Vol. 34, p. 832. a Trans. Am. Ceram. Soc. Vol. 18, p. 192.